This invention relates to a system and method for diagnosing and measuring partial discharge. More particularly, this invention relates to a system and method for diagnosing and measuring partial discharge on-line in a power transmission system and for diagnosing and measuring partial discharge off-line in a high voltage device.
Power transmission systems often suffer from partial discharges due, for example, to faults in the insulation in a high voltage device in the system, loose terminal connections, floating particles, discharges from the transmission lines, etc. These discharges typically result from an electrical stress which locally exceeds a critical value of the dielectric system within the high voltage device. The discharge is considered to be "partial" because it does not bridge the entire dielectric system, causing an undesirable power arc, but merely discharges a small volume of the dielectric system.
Though a partial discharge has a small energy content, it can cause progressive deterioration of the insulation in the high voltage device. If the discharge is continuous, eventually it will be destructive to at least the part of the dielectric system affected by the discharge. Also, the partial discharge can spread and destroy the entire high voltage device. Therefore, it is important to detect partial discharge and determine its source.
Techniques have been proposed to diagnose and measure partial discharge to prevent destruction of the high voltage device. Conventionally, partial discharge is not measured while the high voltage device is installed, i.e., in service, in the power transmission system, due to noise produced by the surrounding equipment and transmitted by the transmission line. Instead, the high voltage device is typically taken off-line, i.e., disconnected from the transmission line, and powered by a motor and generator connected through a regulating transformer.
A conventional test arrangement for measuring partial discharge is illustrated in FIG. 1. The conventional test arrangement includes a Motor 170, a High Frequency Generator Set 160, a Regulating Transformer 110, and Reactors 120. The Regulating Transformer 110 and the Reactors 120 are typically contained in a Mobile Trailer 100, and the High Frequency Generator Set 160 and the Motor 170 are typically housed in a Mobile Trailer 150. In preparation for the conventional test, a high voltage device, e.g., a Power Transformer 300, is taken off-line, i.e. the high voltage transmission line supplying power to the Power Transformer 300 is disconnected. The Power Transformer 300 is connected to the Regulating Transformer 110 via the Reactors 120 and transmission lines 180. During the test, the Power Transformer 300 is powered by the Motor 170 and the High Frequency Generator Set 160, via the Regulating Transformer 110, the Reactors 120, and the transmission lines 180. The High Frequency Generator Set 160 typically operates between approximately 240 and 400 Hz, and the Motor 170 is typically a diesel motor or a motor powered by an external power source (not shown). An output at a bushing tap of the Power Transformer 300 is detected by a Measurement System 190. The Measurement System 190 measures the level of the output, and an operator determines whether or not partial discharge is occurring at that bushing tap or terminal of the Power Transformer 300, based on the measured level.
The Measurement System 190 is typically only capable of measuring the level of an output from a bushing tap of the Power Transformer 300 during a single phase at a time. To measure the level of output during a different phase, the Measurement System 190 must be manually switched.
The conventional test arrangement depicted in FIG. 1 requires a few days and several trained operators to set up, perform measurements, and disconnect. During the time of the test, the Power Transformer 300 is unavailable for power transmission. Since a high voltage device, such as the Power Transformer 300, is a key part of an overall power transmission system, the conventional arrangement for measuring partial discharge can cause power supply interruption, particularly for large high voltage devices. In addition, the conventional arrangement requires bulky equipment. Thus, the conventional test arrangement is expensive, inefficient, and inconvenient.
There is thus a need for a method and system for diagnosing and measuring partial discharge which overcomes the drawbacks of the prior art.